Short take-off and vertical landing ("STOVL") military aircraft, also known as vertical and/or short takeoff and landing ("V/STOL", "VTOL", or "STOL") aircraft (hereinafter all of which are referred to as a "STOVL" aircraft for convenience), are used when a single aircraft is needed to attain both horizontal and vertical flight modes. The vertical flight modes comprise aircraft takeoffs and landings from, e.g., aircraft carriers or other limited-length runways. Such aircraft generally include one or more conventional gas turbine engines that power the aircraft in both the horizontal and vertical flight modes.
To achieve the vertical thrust necessary for vertical flight modes, each engine on the STOVL aircraft may be coupled in some manner to one or more auxiliary lift fans. The lift fans may also be used to control the attitude (i.e., pitch, roll, yaw) of the aircraft. The lift fans are typically disposed within the aircraft fuselage separate from the engine. The primary airflow axis of the lift fan is oriented vertically within the aircraft (i.e., the fan exhaust is pointed downward to generate vertical lift), while the primary airflow axis of the engine is oriented in the conventional horizontal direction. However, the engine exhaust is typically adjustable in a well-known manner from a horizontal position for normal horizontal flight to a vertical position for vertical flight modes. This way, the direction of the thrust produced by the engine may also be varied between horizontal and vertical.
The lift fan may be selectively aerodynamically coupled to the fan exhaust or turbine exhaust of the engine, or the lift fan may be selectively mechanically coupled to the fan or low rotor spool of the engine by a rotating drive shaft. In the latter case, a clutch and gearbox mechanism is usually employed to selectively engage and disengage the lift fan with the gas turbine engine. Examples of these types of well-known propulsion systems for STOVL aircraft are given in U.S. Pat. Nos. 5,464,175, 5,312,069, 5,275,356, 5,209,428, which are all hereby incorporated herein by reference.
The conventional thrust response control system for a gas turbine engine consists predominantly of fuel flow scheduling. Increasing or decreasing fuel flow results in a corresponding increase or decrease in the speed of both the high and low rotor spools within the engine. Fuel flow is typically changed in response to pilot-initiated movements of the throttle or power lever. Also, changing fuel flow typically changes the angle of the inlet guide vanes on the fan and compressor as well as the area of the exhaust nozzle (i.e., jet area). By increasing low rotor speed, the fan draws in more airflow and the engine, in turn, generates more thrust.
However, in a STOVL aircraft having a lift fan mechanically coupled by a drive shaft to the engine fan spool and operating in a vertical flight mode, a significant increase in the inertia (i.e., resistance to speed change) of the lift fan and low rotor spool combination results. This is as compared to the inertia of the low rotor spool alone (i.e., without the lift fan coupled to the low rotor spool). The increased inertia causes a corresponding decrease in the thrust response time of the lift fan and low rotor spool combination when the conventional fuel flow scheduling is employed as the thrust response control system. This decreased thrust response could cause the STOVL aircraft to not be able to meet certain aircraft stability, maneuverability and thrust control requirements.